USB (Universal Serial Bus) standards (USB 1.1 (see “Universal Serial Bus Specification Revision 1.1, Sep. 23, 1998”), USB 2.0 (see “Universal Serial Bus Specification Revision 2.0, Apr. 27, 2000”)) relate to communication interfaces between personal computers and peripherals. A communication interface that complies with the USB standard shall be referred to as a “USB interface” below.
A USB interface has a connector for D+ and D− lines, a VBUS line and a GND line. The D+ and D− lines are wires for data transfer, and the VBUS and GND lines are lines for supplying power. The VBUS line is limited to a current of 100 to 500 mA.
A bus-powered device and a self-powered device are stipulated by the USB standard. A bus-powered device is a device that operates on power supplied from a host or USB hub via a USB interface. A self-powered device is a device that operates on power supplied from its own main power source (an AC power source or battery).
For a description of prior art relating to USB power management, see the specification of Japanese Patent Application Laid-Open No. 2001-75682. This patent reference describes art whereby the power supplied to a device that exceeds the power supply capacity of a host is limited when the device is connected.
However, a conventional image sensing apparatus (such as a digital video camera or digital camera) consumes a large amount of power and cannot operate solely on power supplied from a USB interface. In other words, there is not a single conventional image sensing apparatus that can operate as a bus-powered device. This problem also arises with an image sensing apparatus having a communication interface compliant with a standard similar to the USB standard.